The invention relates to an installation for the flocculation of water or sludge loaded with suspended matter, such as for example sludge from dredging, from the mineral, urban or paper and pulp industry. It also relates to a flocculation method implementing the said installation.
Sludges can be flocculated by means of high molecular weight polymers and in particular by means of polyacrylamide. These polymers are water-soluble polymers generally used in powder form whereof the main drawback is that they dissolve in water with difficulty.
In most cases, particularly for low and medium consumption, the polymer is dispersed in the dissolution water using an ejector or a wetting apparatus avoiding the formation of lumps, called “fisheyes”. The solution concentration remains low for high molecular weight products, and this dissolution must take place in tanks of sufficient size to obtain complete dissolution.
In certain cases, the polymer powder is injected directly into the water of the sludge to be treated, at the head of long transport lines, particularly in hot suspensions (30 to 60° C.). The powder dissolves during the transport and flocculates the solids. However, due to turbulence, the flocs are mechanically broken, causing high extra consumption of polymer. The installation is simple, but the yield is low.
Another solution is to avoid the complete dissolution of the polymer. In this case, the aggregation of the chains leads to apparent molecular weights much higher than the true molecular weights of the polymer. This is used especially for the flocculation of municipal sludge on centrifuge. In this case—the high concentration of the polymeric solution (5-10 g/litre of polymer for example)—the short dilution time (15 to 30 minutes)—and the very fast contacting time with the suspension (before or at the time of the polymer injection into the water to be treated) prevent the separation of the macromolecular chains, thereby leading to an increase in the size and strength of the flocs. A person skilled in the art has found in particular that the molecular aggregates obtained have the same characteristics as if use had been made of completely dissolved higher molecular weight polymers.
One alternative solution is to completely dissolve the polymer before injecting it into the suspension to be treated. In this case, dissolution generally takes place with polymer solutions in concentrations of 1 g/l for treating water and 3 g/l for sludge. Conventionally, this dissolution has the major drawbacks of requiring:                the use of large size and costly equipment, particularly and usually with two tanks, one for preparation and one for maturation/injection,        and preparation (residence) times that are generally longer than 1 hour.        
Documents U.S. Pat. No. 4,874,588 and U.S. Pat. No. 4,845,192 describe a polymer dispersion apparatus which reduces the size of the powder to 100-200 microns and thereby shortens the dissolution time. However, this apparatus has the defect of clogging rapidly due to the consistency of the polyacrylamide which becomes particularly viscous in contact with the water and forms aggregates. To prevent this from occurring, it is necessary to rotate the grinder at very high speed ((10-13000 rpm) for a 200 mm ring) in order to self-clean the apparatus by centrifugal action. This method is not widespread industrially due to the technology of the apparatus and its limited throughput. In fact, at a maximum water throughput of 10 m3/h with a 200 mm ring in a concentration of 0.5%, only 50 kg/h of polymer is batched with problems of grid clogging and very short apparatus service life.
The problem that the invention therefore proposes to solve is to develop an installation for the flocculation of sludge loaded with suspended matter that uses a dispersion apparatus of the same type as the one previously described and which allows to disperse large quantities of polymer, in a high concentration, without the risk of clogging the grinding device.